Java Classes and Java Objects

Classes >
Siva Nookala - 22 Mar 2016

Here are few points which we need to understand about classes and objects.

A class is like a blue print and we can create as many objects using that class.

All the objects of one class will have same properties and they will be copied from that class

We can create multiple objects of a class and multiple objects can be of same class

Every object should belong to a class, since we can not create objects with out a class

A class is also referred as type and an object is also referred as instance.

To understand the difference between a class and objects, let us look at the following examples.

Class	Object	Properties	Description
Brick Mold	Bricks	Width, Breadth, Height	Using one brick mold we can create as many bricks. Every brick has the same width, breadth and height as the mold.
Refrigerator Ice Tray	Ice cubes	Width, Breadth, Height	Similar to brick mold, the size of the ice cube is dependent on the tray
Original Examination Form	Photo Copied Examination Forms	Fields like Name, School, Percentage, Subjects and any instructions and guidelines for filling the form	We can take as many copies of Examination forms using the original and every copy has same fields, instructions and guidelines as the original form.
House Plan	Houses	Number of bedrooms, kitchen size, toilets, hall size, appliances location etc	We can construct multiple houses using a plan and every house will have the same number of bedrooms, kitchen size etc.
Student	Mahesh, NTR, Prabhas, Arjun	Name, Marks, Section, Address, Mobile Number	Every object created using the Student class will have the same properties.

Compared to the creation of primitive types, creating an instance (or object) of Student class (or type) is different. We have to use the new keyword for creating an object of non-primitive data type or user defined data type. For e.g., in primitive types, the moment a variable is declared, the space required for that variable is also allocated.

int a = 5;

This declares the variable a and allocates 4 bytes for that integer. But with classes (or non-primitive data types), the allocation of the space required for that object has to be done using new keyword as shown below. If the class Student is declared like this

class Student
{
    String name;
    int marks;
    char section;
}

we can create object for student Mahesh like this.

Student mahesh = new Student();

Similarly, we can also create student objects like ntr, prabhas and arjun as shown below.

Student ntr = new Student();
Student prabhas = new Student();
Student arjun = new Student();

Above, the right side of equals shows how to create objects and ntr, prabhas and arjun are called references. We will discuss more about references in Java Objects References. Classes-objects

Java Class

Java Objects References